1. Field of Invention
This invention relates to an electrically alterable read-only memory and more particularly to an amorphous semiconductor memory device cell for employment therein.
2. Description of the Prior Art
Most semiconductor memory cells are volatile in the sense that they must be refreshed in order to maintain the data stored therein. In the case of semiconductor memory latches, the cell is still volatile in the sense that data is lost should there be a power failure. However, it has been discovered that certain amorphous semiconductor materials are capable of being switched to and from a low resistance crystalline state which does provide a non-volatile memory cell. Particular materials that may be employed are disclosed in the Ovshinsky U.S. Pat. No. 3,271,591, the Neale U.S. Pat. No. 3,699,543 and the Buckley U.S. Pat. No. 3,886,577. A particular type of memory switching amorphous semiconductor material is the tellurium based chalcogenide class materials which have the general formula; EQU Ge.sub.A Te.sub.B X.sub.C Y.sub.D
such amorphous high resistance semiconductor material can be placed between a pair of spaced apart electrodes such that the application to one of those electrodes of a voltage pulse above a given threshold produces a relatively low resistance filamentous crystalline path (set operation). A reset set of pulses of appropriate value and duration causes the crystalline path to return to the relatively amorphous state (reset operation).
The prior art designs of amorphous or ovonic memory switches have had a characteristic threshold voltage V.sub.T which is high at the first operation and in early operating life and lower thereafter ("first-fire effect") or which declines continuously throughout the life of the switch. Particularly, this decline is in response to repeated "reset" operations where the memory element is restored from its conducting condition to its high resistance condition. However, there are instances where it appears that the device lasted through 10.sup.6 set-reset cycles where the threshold voltage was observed to have a minimum low value (between 5 and 10 volts) and was relatively invariant to additional write cycles.
It now appears that electromigration of the constituents of the memory material toward the different electrodes causes the steady decline in the threshold voltage. In the above described materials, germanium is shown to migrate to the negative electrode and to approach 50% concentration there. Similarly, tellurium migrates to the positive electrode. This migration of material produces regions that are inactive in the switching process because their ratios of constituents are no longer appropriate. The region where the ratio of constituents is appropriate for switching is thus reduced in effective thickness and the threshold voltage becomes low, similar to that of a much thinner layer.
The migration of material also produces concentration gradients. Diffusion then operates as a countervailing process, producing an equilibrium. Thermal gradients may also contribute to the process.
The above referenced Buckley patent discloses an ovonic memory structure in which the threshold voltage decline is altered by placement of a tellurium layer between the positive electrode and the amorphous memory material layer. This alters the threshold voltage decline but doesn't eliminate it.
It is then an object of the present invention to provide an improved amorphous semiconductor memory device.
It is another object of the present invention to provide an improved amorphous semiconductor memory device which is not characterized by a declining threshold voltage.
It is still another object of the present invention to provide an amorphous semiconductor memory device which has a relatively constant threshold value over a large number of set-reset cycles.